1. Field of the Invention
The present invention relates in general to cooling the leading and trailing edges and tips of an airfoil by conduction and relates particularly to an airfoil having a core body cast from a nickel-based superalloy and having leading and trailing edges formed of nickel aluminide bonded to the core body.
2. Description of Prior Developments
Thermal loads applied to the leading and trailing edges and tips of a gas turbine engine airfoil can adversely affect the airfoil's useful life. These thermal loads are particularly troublesome along the trailing edge region because, in order to produce an aerodynamically efficient airfoil, the trailing edge region is typically designed with a relatively thin cross section, This thin section limits the space available for efficiently removing the heat from this region using conventional convection and conduction cooling techniques.
Moreover, conventional nickel-based superalloys such as N5 and N6 usually exhibit relatively low thermal conductivity, approximately 13 BTU/Hr/Ft/F.degree., This low thermal conductivity also compromises the ability to efficiently cool the airfoil.
Because the heat loads applied to the pressure and suction surfaces of a gas turbine airfoil cannot always be efficiently removed by convection cooling and conduction cooling, the trailing edge of the airfoil typically experiences high operating temperatures. This can often become the limiting factor in the life of the airfoil.
Although some prior designs have bonded ceramic inserts within the hottest regions of an otherwise metallic airfoil to withstand high operating temperatures, the ceramic-to-metal bonds experience significant problems due to the high degree of differential thermal expansion across the bonds. Moreover, rather than transfer heat away from the hot regions of the airfoil, the ceramic inserts simply withstand the temperatures. Heat is not efficiently transferred from the ceramic inserts since ceramic materials exhibit low thermal conductivity values, i.e. low coefficients of heat transfer.
Even though there is generally sufficient internal space available for effectively cooling the leading edge region of an airfoil by convective cooling, the high heat loads applied on the leading edge stagnation point often produce large temperature gradients. These gradients extend between the hot leading edge of the airfoil and the cooler adjacent internal reinforcing web which defines a first cooling cavity next to the leading edge. The thermal stresses produced by these thermal gradients adversely affect the life of the airfoil.
One method of reducing a thermal gradient across a relatively small region is to transfer the heat from the region by conduction with a highly heat conductive material. Unfortunately, in the case of gas turbine engine airfoils, the currently available materials which can withstand the high operating temperatures of the hot flowing gasses and which are highly heat conductive are not as strong as the conventional nickel-based superalloys presently used to produce conventional airfoils.
For example, nickel-aluminide materials which are highly heat conductive are not by themselves presently strong enough to withstand the loads typically applied to gas turbine engine airfoils such as rotor blades and stator vanes. Even though NiAl with a K value of about 40 BTU/Hr/Ft/F.degree. exhibits a threefold increase in heat conductivity over conventional nickel-based superalloys such as N5 and N6, NiAl is simply not strong enough to use throughout the entire airfoil.
A need therefore exists for an airfoil design which can exploit the advantages of the high heat conductivity of nickel aluminide (NiAl, NiAl.sub.3) yet which can maintain the strength of conventional nickel-based superalloys over the majority of the airfoil. A particular need exists for applying the heat conducting abilities of NiAl and NiAl.sub.3 to the leading and trailing edges of an airfoil where thermal loading can produce the most adverse affects oh the life of the airfoil.